As an airship moves between ground level and high altitude, various thermal changes are encountered due to the fact that external atmospheric temperature decreases as altitude increases. Thus, as the airship ascends from ground level, the temperature decreases resulting in a cooling of the walls of the hull or envelope of the airship. The rate at which the temperature decreases with increased altitude is referred to as the temperature lapse rate. As the walls of the hull decrease in temperature, sites for water condensation are created. Furthermore, as the air within the envelope continues to cool down to its dew point temperature, the water vapor in the air contained in the hull of the airship begins to condense into water. As this condensed water accumulates it may contribute to several unwanted problems discussed below.
To control the descent and ascent of an airship, conventional airships generally use ballonets or storage balloons contained within the envelope of the conventional airship. These ballonets separate the helium volume of the airship from the air volume. In addition, the ballonets, which have a much smaller volume than that of the envelope, inflate and deflate with air to control the trim and buoyancy of the airship. In fact, during descent of the airship from altitude, atmospheric air, external to the airship, is blown into the ballonets to bring the airship to the ground. Thus, conventional airships, because of the relatively small size of the ballonets, have a small quantity of air that includes water vapor. As such, the water vapor contributes only a small percentage of potential additional weight to the payload of the conventional airship. Since conventional airships and aerostats fly at relatively low altitudes, the change in temperature due to the temperature lapse rate is relatively small, which minimizes the amount of water condensation that forms on the interior surfaces of the air volume of the hull, and reduces the formation of ice fog within the air volumes of the hull as well.
In contrast to a conventional airship, the hull or envelope of a high altitude airship has a much larger air volume than that of the ballonets of a conventional airship. For example, the envelope of a high altitude airship may be on the order of five million cubic feet. In further contrast to the conventional airship, the high altitude airship will experience an extreme decrease in external air temperature due to the temperature lapse rate as it ascends from ground level to flight altitude. In similar fashion to a conventional airship, the high altitude airship uses external atmospheric air that is blown into the envelope to decrease buoyancy, causing the high altitude airship to descend. As the high altitude airship descends and intakes external ambient air, the water content of the air represents a significant mass addition, if condensed.
Water vapor within the hull of the airship may condense into either liquid or solid form in several manners, as the external air temperature decreases during the ascent of the airship. For example, water can condense on the inside surface of the hull or envelope of the airship, as the envelope temperature decreases due to the change in external air temperature and radiation of heat from the hull. As the internal surface of the envelope cools below the dew point or frost point of the internal air volume, liquid water or ice will form on the inside surfaces of the envelope. This can occur on the surfaces of structures mounted to the envelope of the airship that are exposed to the internal air volume, such as air blowers, air valves, check valves and air sensors. In addition, in the bulk internal air volume of the envelope, water can condense in the form of liquid water droplets. These water droplets tend to settle out on the internal surfaces of the envelope in liquid form. Yet another manner in which water can condense within the envelope of the high altitude airship is in the form of ice crystals. The formation of ice crystals occurs when the internal water partial pressure and air temperature are such that the water can convert directly (sublimate) from its vapor phase to its solid phase in the form of extremely fine ice crystals, sometimes referred to as ice fog. It should be noted that the ice crystals of ice fog tend to remain suspended in the air volume of the envelope of the airship.
High altitude airships may use a number of cells within the envelope to separate the lifting gas, such as helium, from the air that is used to maintain the shape and rigidity of the airship at altitudes below flight altitude, and to control the buoyancy of the airship to perform ascents and descents. The cells comprise a flexible and pliable material used to retain the lifting gas in discrete regions. As the high altitude airship ascends and descends, the material of the cells deflates or inflates in accordance with the change in altitude of the high altitude airship. Thus, during an ascent, the flexible and pliable material of the cells changes shape, which may result in the trapping of condensed water between the material of the cell and the inner wall of the envelope of the high altitude airship. This entrapped water may be in the form of ice or liquid that condensed upon the interior surfaces of the envelope or other components of the airship, or that has formed in the bulk air volume of the envelope and has settled onto the interior surfaces thereof. Furthermore, because of the large volume of air contained in the envelope, a large quantity of water may be condensed and trapped. Because of the trapping, the water cannot be easily evacuated out of the envelope through the various air valves carried by the envelope. Without any means of evacuating the trapped water, the water may continue to accumulate within the envelope, adding a significant amount of additional weight to the high altitude airship. Moreover, as the high altitude airship continues to ascend, the trapped water may freeze to form ice, which is very difficult to remove from the envelope.
This water content, if allowed to condense within the envelope of the airship during an ascent, could become entrapped and create several potential problems that may affect airship performance. For example, the entrapped water mass may reduce the payload capacity of the airship, reduce the flight altitude of the airship, and/or adversely affect the trim of the airship. Furthermore, the entrapped water can form ice that may interfere with the proper operation of the air pressurization blowers, vent valves, check valves and sensors. Moreover, an uncertainty in the altitude, performance, trim, and control of the airship will result because the mass of the entrapped water will continually vary due to the environmental fluctuations of the ambient external air used within the envelope of the airship.
Therefore, there is a need for a dehumidification system for an airship to prevent water condensation and ice formation within the envelope during ascent of the airship. And there is a need for a dehumidification system for an airship to reduce the amount of water vapor or humidity contained in the envelope of an airship prior to an ascent. Additionally, there is a need for a dehumidification system that is able to lower the dew point of the air contained within the envelope to a point where the water vapor in the air avoids the liquid phase, and sublimates into ice fog, which can be readily removed from the envelope of the airship.